An emissive cathode emits electrons during processes such as field emission, thermionic emission, and photoemission. A key parameter governing electron emission from a cathode is the work function of its emitting surface. A low work function is desirable since it generally corresponds to a high emission current. It is also desirable that cathode performance not degrade substantially upon normal operation, or following exposure to air and/or high temperatures.
In one design approach, the emissive cathode consists of a semiconductor substrate coated with an electropositive metallic layer. For a description of cathodes having a GaAs substrate coated with composite Cs--O films, see the article by Rougeot et al. in Adv. Electronics and Electron Phys., 48: 1-36 (1979). For a description of a cathode having a Cs--O-coated Si substrate, see the article by Levine in Surf. Sci. 1973, p. 90-107.
Diamond and diamond-like carbon have been proposed as potential substrate materials for semiconductor emissive cathodes. The deposition of a durable layer of electropositive metal on carbon-containing or diamond substrates has proven difficult, however. This difficulty stems from the fragility of the attachment of metal atoms to the substrate. As illustrated by Geis et al. in U.S. Pat. No. 5,463,271, herein incorporated by reference, heating a conventional Cs-coated diamond substrate to a moderate temperature (.about.200.degree. C.) reverses the cathode performance to that of untreated diamond (see FIG. 5 of Geis et al.).